A known method for producing filter rods for cigarettes and an apparatus suitable for carrying out this method are described in DE 41 09 603 A1. The known apparatus consists essentially of a treatment unit, in which a supplied filter tow strip is subjected, inter alia, to stretching and fluffing, an auxiliary unit for applying an additional filter material component on the treated filter tow strip, a formatting unit for forming a filter skein by collected and wrapping the treated filter tow strip with a wrapping material, and a cutting unit for successive severing of filter rods from the filter skein. With the known apparatus a single filter skein is produced.
In the treatment unit of the known apparatus a driven brake roll pair is provided, which draws the filter tow strip off a filter tow bale. According to the state of the art there are known also treatment sections which use at the inlet of the treatment section a non-driven brake roll pair. Such a treatment section is for example a commercial AF 2 of Korber AG, Hamburg. The treatment section described in DE 41 09 603 A1 with a driven brake roll pair has the following disadvantages: The farther the filter tow strip is drawn off from the bale, the longer will be the piece of filter tow strip between the bale and the drawing roll pair, owing to which the weight of the piece of filter tow strip between the bale surface and the drawing roll increases and hence the filter tow strip is stretched. In addition, with increasing drag length of the filter tow strip at high drawing speeds, the air friction acting on the filter tow strip becomes greater and greater, which also leads to a prestretching of the filter tow strip. This stretching means, however, that less and less weight of the filter tow strip per unit of time or per unit of length is fed to the drawing roll pair, that is, the fed quantity of filter tow strip varies at constant speed of the brake roll. This has an adverse effect on the produced filter skein, as its density also changes when the supplied amount of filter tow strip changes. If too little filter tow strip is supplied, the density or mass of the produced filter rods or filter skein will be insufficient, so that the produced filter skeins or rods are unusable and constitute scrap. A non-driven brake roll pair or a "dragged" brake roll partly compensates this disadvantage and further fluctuations deriving from the filter skein, e.g. curl index fluctuations, which would affect the quality of the filter skeins. In the cited treatment section AF 2, depending on the specification of the filter tow strip in processing, a brake force to be set at a constant value is applied on the dragged roll pair. This is evident from the brochure "Technische Information 2-01" the "Kabelkennlinie" (Cable Characteristic) of Rhodia AG, Edition January 1989. Control of the mass in the filter skein is obtained according to the state of the art by changing the feed speed of the driven rolls.